metaphase plate. The centromeres着丝粒 finally divide, and each sister chromatid moves to one of the poles of the spindle. The next phase is telophase , followed again by cytokinesis. The result of the entire process is four haploid cells in which parental chromosomes are randomly distributed.
第二次核分裂开始于分裂中期，子细胞中染色体重新排列在赤道板上。着丝粒最终分离，每个姊妹染色单体分向两极。接着胞质分裂。产生4个单倍体，父母染色体随机分配。 Asexual Versus Sexual Reproduction
Mitosis and meiosis, respectively分别, make simple cell division and sexual reproduction possible. Each means of passing on hereditary 遗传 information has advantages. In asexual 无性的 reproduction the parent organism生物体 gives rise to offspring 后代that are genetic clones of the parent. The advantages of this type of reproduction are that it preserves the parent's successful genetic complement 遗传互补, requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction. A major disadvantage of the asexual mode is that a single catastrophic灾难的 event or disease may destroy an entire population of genetically identical organisms. A prime benefit of sexual reproduction is that it provides genetic variability and a ready mechanism机制 for the elimination of deleterious mutations. It also allows "new" gene forms to arise and spread through populations.
有丝分裂和减数分裂在传递遗传信息过程中各有优势。体细胞的繁殖就是父母本的克隆，其优势是保留了父母本的成功遗传信息，不需要特殊器官，比性复制快的多。但一个简单灾难性事件或疾病都可能摧毁一个细胞群体。性复制的优势是它提供了遗传可变性和现存排除有害突变的机制。也可以产生新的基因并在种群中蔓延。
课后作业：第一篇阅读材料
答案：1C,2B,3D,4B,5A,6D
Glossary
mitosis 有丝分裂
Process that results in equal and identical distribution of replicated chromosomes into two newly formed nuclei.
meiosis 减数分裂
The process by which a nucleus divides into four daughter nuclei, each containing half the number of chromosomes of the parent nucleus.
histone 组蛋白
One of a group of simple proteins that have a high content of the basic amino acids arginine or lysine and are found associated with nucleic acids in the chromatin of eukaryotic cells.
nucleosome 核小体
The basic unit of chromatin structure in eukaryotic cells. A nucleosome consists of eight histone molecules of four different types, together with about 140 base pairs of DNA coiled around it.
chromatin 染色质
The material of which the chromosomes are composed. DNA and protein are the main constituents of chromatin, most of the protein being histone.
karyotype 核型，染色体组型 ,
The appearance of the chromosome complement of an organism or cell.
homologous pair 同源染色体对
Chromosomes that pair during meiosis. Each homologue is a duplicate of one of the
chromosomes contributed at syngamy by the mother or father. Homologous chromosomes contain the same linear sequence of genes and as a consequence each gene is present in duplicate. diploid二倍体